Wavelength division multiplex optical telecommunications network

ABSTRACT

A wavelength division multiplex (WDM) optical telecommunications network comprises a plurality of nodes interconnected by optical fiber waveguides. Communication traffic is communicated between nodes by optical radiation modulated with the communication traffic, the radiation being partitioned into aplurality (N) of wavelength channels each channel having a discrete non-overlapping waveband. Each node drops at least one of the wavelength channels to thereby define a connection to the node and adds at least one wavelength channel to the network to define a connection to another node of the network. The network is characterized is that at least one node has ascribed to it a fixed subset of the N wavelength channels and drops each of the wavelength channels of the fixed subset from the network. The node further comprises a wavelength selectable transmitter capable of adding one or more of at least any of the remaining wavelength channels other than the subset to the network to define a connection to another node of the network.

This invention relates to wavelength division multiplex (WDM) opticaltelecommunications network and more especially to a re-configurable WDMtelecommunications network

As is known, WDM optical telecommunications network comprise a pluralityof spatially disposed nodes which are interconnected by optical fiberwaveguides. Communication traffic is communicated between the nodes byoptical radiation modulated by the communication traffic which isconveyed by the optical firers.

Optical radiation in the context of the present invention is defined aselectromagnetic radiation having a free-space wavelength of 500 nm to3000 nm, although a free-space of 1530 nm to 1570 nm is a preferred partof this range. In wavelength division multiplexing, the opticalradiation is partitioned into a plurality of discrete non-overlappingwavebands, termed wavelength channels or optical channels, and eachwavelength channel is modulated by a respective communication channel.

The connectivity between nodes, in terms of the routing of communicationtraffic, is determined by the waveband (wavelength) of the wavelengthchannel. Typically a single wavelength channel is ascribed to uniquelydefine a given connection between two nodes though it is known to usemore than one wavelength channel for the same connection to increasetransmission capacity.

One network topography, a ring configuration, is one in which the nodesare connected by the optical firers in a point-to-point serial manner toform an unbroken (closed) loop or ring configuration. At each node oneor more drop optical (wavelength selective) filters are connected inseries within the optical fiber ring and each removes (drops) a singlewavelength channel from the WDM radiation passing around the ring whilstallowing the remainder of the wavelength channels to pass substantiallyunattenuated. Additionally, at each node, wavelength channels can beadded to enable communication with other nodes.

WDM network configurations can comprise a full mesh in which every node,in terms of wavelength channel connection, is connectable to every othernode, hub networks in which one node, termed a hub, is connected toevery other node or linear networks in which nodes are connected inlinear manner.

In general optical ring networks can be divided into passive networkswhich do not include optical amplification within the ring or at thenodes and active networks which include optical amplifiers (typicallyErbium doped fiber amplifiers EDFAS) to compensate for loss within thenetwork. The former are typically limited to a few tens of kilometesaround the ring and are often used as part of local area networks, andare termed “metro” (metropolitan) networks. In order to keep cost to aminimum metro networks are generally non re-configurable (i.e. fixedwavelength allocation) systems in which the intercormection of nodes,which is determined by the wavelength channel allocation, is fixed frominstallation of the system. The PMM series of networks sold by MarconiCommunications Limited of Coventry, England is an example of a nonre-configurable metro network and is of the order of 10-200 km incircumference. The larger end of the range is an active networkincluding EDFA amplification. The smaller end of the range is a passivenetwork. In such a network wavelength channels are dropped at respectivenodes using drop filters (dielectric interference filters) which have afixed wavelength passband corresponding to the waveband of thewavelength channel it is designed to drop. Since the drop filters have afixed wavelength characteristic, the wavelength channel allocation, andhence interconnection of nodes, cannot be re-configured. The only way tore-configure such a system would be to physically change the dropfilters at each node, requiring a site attendance which is both timeconsuming and expensive. This is a notable disadvantage since metronetworks are typically installed in locations such as business parks andlarge office blocks where there is frequently a need to re-configure thenetwork, for example to change the transmission capacity ofinterconnections or to add or remove users.

Re-configurable WDM optical telecommunications networks are also knownin the art One example is the PMA32 network produced by MarconiCommunications Limited of Coventry, England. This network is a thirtytwo wavelength channel network that is fully re-configurable in thateach node can be remotely re-configured to selectively add and/or dropany one of the thirty two wavelength channels. Such Re-configurabilityis achieved through the use of wavelength tuneable drop filters andwavelength tuneable optical transmitters for adding channels.

Fully re-configurable WDM networks are ideally suited for largenetworks, for example regional networks or networks handling traffic fora large city, though they are costly since every node has to havehardware which is capable of handling (dropping and/or adding) each ofthe wavelength channels supported by the network.

Although a fully re-configurable network, such as those described above,could be used in the metro environment, the cost would be prohibitivelyexpensive and would also give a degree of flexibility beyond that whichis usually required.

The inventors have appreciated that a need exists therefore for a WDMnetwork which is at least partially re-configurable and which iseconomically viable for use in the metro environment.

In its broadest form, the invention resides in at least some of thenodes being configured to drop a fixed subset of the total number ofwavelength channels supported by the WDM network and additionally havingthe capability of being able to selectively add any of the remainingwavelength channels.

More specifically, according to a first aspect of the invention there isprovided a wavelength division multiplex (WDM) opticaltelecommunications network comprising a plurality of nodesinterconnected by optical fiber waveguides, in which communicationtraffic is communicated between nodes by optical radiation modulatedwith the communication traffic, said radiation being partitioned into aplurality (N) of wavelength channels each channel having a discretenon-overlapping waveband (λ₁ to λ_(N)), wherein each node includes meansfor dropping at least one of the wavelength channels to thereby define aconnection to said node and includes means for adding at least onewavelength channel to the network to define a connection to another nodeof the network, the network being characterized by: at least one nodehaving ascribed to it a fixed subset (M_(i)) of the N wavelengthchannels and including means for dropping each of the wavelengthchannels of the fixed subset (M_(i)) from the network; and wavelengthselectable means capable of adding one or more of at least any of theremaining wavelength channels other than the subset to the network todefine a connection to another node of the network.

The present invention provide the advantage of a low-cost partiallyre-configurable network which is particularly suited to metro typenetworks which can be ring type or linear. However, the invention isalso applicable to other network types.

Advantageously every node has ascribed to it a respective fixed subsetof the total wavelength channels and includes means for dropping each ofthese wavelength channels from the network; and every node furtherincludes wavelength selectable means for adding one or more of at leastany of the remaining wavelength channels other than the respectivesubset to the network.

The means for adding the one or more wavelength channels preferablyincludes at least one wavelength selectable transmitter. Advantageouslythe, or each, wavelength tuneable transmitter is wavelength tuneable totransmit on any of the N wavelengths supported by the network Such anarrangement enables the same transmitter to be used at any node of thenetwork. Advantageously the, or each, wavelength selectable transmittercomprises a tuneable laser such as a multi-stage wavelength tuneablelaser. Preferably, the, or each, transmitter further comprises avariable optical attenuator (VOA) for controlling the power of thewavelength channel. Such an arrangement can be further set to maximumattenuation during wavelength tuning. The use of wavelength tuneabletransmitters, which can be adjusted remotely, enables re-configurationof the network without the need for on-site engineers. This greatlyreduces network maintenance costs.

In one configuration the wavelength channels of the fixed subset arecontiguous and comprise a band of consecutive wavelength channels. Suchan arrangement is beneficial in that the means for dropping the subsetof wavelength channels can then conveniently comprise a broadband dropfilter.

In an alternative configuration WDM network according the wavelengthchannels of the subset comprise a plurality of discrete wavelengthchannels which are non-contiguous. Preferably the means for dropping thesubset of wavelength channels then comprises one or more fixedwavelength selective filters. In one arrangement the one more filtersare configured to drop every Q^(th) (Q>1) wavelength channel.

Advantageously the node further comprises means for separating therespective wavelength channels of the subset. Such means can comprises ade-multiplexer such as an array waveguide grating or alternatively anmulti-way (m-way) splitter and m optical filters to select a respectiveone of the wavelength channels of the subset.

Preferably the number of wavelength channels that can be added at thenode is equal to the number of wavelength channels dropped at the node.

The network of the present invention finds particular application to anetwork in which the nodes are interconnected in a ring configuration.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 shows a node in accordance with a first embodiment of theinvention for use in a WDM optical telecommunications network;

FIG. 2 shows a node in accordance with a second embodiment of theinvention;

FIG. 3 a shows a node in accordance with a third embodiment of theinvention;

FIG. 3 b shows an alternative implementation of the node of FIG. 3 a;

FIG. 4 a shows a node in accordance with the invention with a sub-ringsubtending from the node;

FIG. 4 b shows an alternative realization of the embodiment of FIG. 4 a;

FIG. 4 c shows a further alternative realization of the embodiment ofFIG. 4 a; and

FIG. 5 shows a WDM ring telecommunications network embodying theinvention.

Whilst each of the embodiments of the present invention to be describedare primarily intended for use in metro WDM telecommunications network,passive or active, it should be understood that the invention is equallyapplicable to any type of WDM network such as regional ring networks orlinear networks. In general metro networks have a ring configuration inwhich the nodes are serially connected by optical firers to form aclosed loop or ring. The network can be protected or unprotected. In aprotected network the nodes are connected by two separate optical fiberrings and WDM radiation is transmitted around the rings in clockwise andcounter-clockwise directions respectively. As is known such anarrangement ensures a protection path is available in the event of afiber break occurring. In the following description of the invention,and for the sake of brevity, an unprotected network is described inwhich there is only a single optical fiber connection between the nodes.The skilled person will, however, appreciate that the invention can bereadily applied to a protected network.

All the embodiments to be described provide a partially re-configurableWDM network. The WDM network of the invention comprises n nodes and hasN wavelength channels (λ₁ to λ_(N)). At least one, preferably all, ofthe models i (i=1 to n) has ascribed to it a fixed subset M_(i) of thetotal N wavelengths channels carried by the network and includes meansfor dropping each of these fixed wavelength channels from the network.The subset ascribed to each node is unique to the node and comprises aplurality of wavelength channels. Additionally this/these models is/arecapable of selectively adding any of the remaining N wavelength channelsother than the subset of wavelength channels being dropped at the node(addition at a node of one of the wavelength channel which is one of thesubset would define a connection to the same node). The subset M_(i) ofwavelength channels dropped at each of these nodes remains fixed andthus defines a fixed number m (equal to the number of wavelengthchannels in the subset M_(i)) of connections to the node. The ability tobe able to selectively add any of the remaining wavelength channelsenables connections between nodes to be re-configured. A connection to agiven node is established by selecting one of wavelength channels of thesubset M_(i) of the given node. Since number m of possible connectionsto each node is fixed, the system is partially re-configurable. Such asystem is much cheaper than a fully re-configurable network since eachnode need only include hardware capable of dropping a fixed subset M_(i)of the total number of wavelength channels.

In each of the embodiments, the subset M_(i) of wavelength channelspreferably comprises groupings of consecutive (adjacent) wavelengthchannels (i.e. bands of wavelength channels) thereby enabling the use ofbroadband filters to drop the subset of wavelength channels at eachnode. These filters may be dielectric filters or fiber Bragg gratingfilters for example. A series of filters could be used in place of abroadband filter though this is not favoured as the optical insertionlosses (express path loss through the node) are too high. The wavelengthchannels of the subset can alternatively comprise different groupings ofnon-consecutive channels.

FIG. 1 shows a network node 2 embodying the invention. The node 2 isintended, for example, for use in a thirty two (N=32) wavelength channel(λ₁ to λ₃₂) WDM metro ring network operating at C-band (1530-1560 nm).In this embodiment, and in each of the following embodiments an opticalsupervisory channel (OSC) λ_(OSC)=1510 nm is used to support inter-nodecommunications for network control and administration purposes. As isknown the OSC does not carry communication traffic. Inter-nodecommunications can alternatively be achieved by use of in-bandmanagement channels or by other means.

The node 2 comprises, serially connected between a network optical fiber4 for receiving WDM radiation at the node and a network optical fiber 6for outputting WDM radiation: a three port filter 8 for dropping theOSC, an optical amplifier (EDPA) 10, an optical drop filter 12, anoptical coupler 14 for enabling wavelength channels to be added at thenode 2, a second optical amplifier (EDFA) 16, and a second coupler 18for adding the OSC to the WDM radiation output along the optical fiber6. The OSC is dropped before the input of the EDFA 10 and added afterthe output of the EDFA 16 to maintain inter-node communication in theevent of failure of either of the EDFAs. The optical amplifiers 10 and16 compensate for inter-node radiation losses which are caused, forexample, by optical fiber losses, filters, connectors and othercomponents present in the optical path inter-connecting the nodes. Suchoptical amplification is of course optional and would be omitted in apassive network

For ease of manufacture the three port filter 8 and the first opticalamplifier 10 are located on a receive (RX) line card 20; the opticaldrop filter 12 and coupler 14 are located on a card 22; and the opticalamplifier 16 and the optical coupler are located on a transmit (TX) linecard 24. The cards 20, 22, 24 are denoted by dashed lines in the Figure.

The optical drop filter 12 is preferably a three port filter and isconfigured to drop a fixed subset M_(i) of the total number ofwavelength channels N to a first optical output port whilst allowing theremainder of the wavelength channels to pass substantially unattenuatedto a second optical output port which is connected to a first input ofthe coupler 14. The filter 12 is a broadband filter having a fixedwavelength characteristic which drops a number m, four in this example,consecutive (adjacent/neighboring) wavelength channels M_(i)=λ_(a) toλ_(d).

The card 22 additionally includes an optical de-multiplexer 26 (arraywaveguide grating AWG) whose input is connected to the first output ofthe optical drop filter 12 and which passively separates the subsetM_(i) of wavelength channels (λ_(a) to λ_(d)) into respective channelsand routes each wavelength channel (λ_(a) to λ_(d)) along a respectiveoptical path to a respective receiver 28. Moreover the card 22 includesa passive multi-input (p-way) optical combiner 30 (cascaded fused fibercoupler) for combining wavelength channels generated by a plurality p ofoptical transmitters 32. The output of the combiner 30 is connected to asecond input of the optical coupler 14 which adds the radiation from thetransmitters 32 to the WDM radiation passing through the node.

Typically the transmitters 32 comprise wavelength tuneable lasers whichare capable of operating at any of the N wavelengths (λ₁ to λ_(N)) ofthe network. (Although it is unlikely that the transmitters will everoperated at any of the wavelengths of the respective subset M_(i) it ispreferred that each is capable of operation at all N wavelengths toenable the use of a single transmitter at any node). Each transmitter 32preferably includes a variable optical attenuator (VOA) 34 forcontrolling the power of the wavelength channel being added to thenetwork. Additionally each VOA 34 is preferably set to maximumattenuation during tuning, power-up and down of the laser to preventoutput radiation being added to the network.

In operation the optical filter 12 drops the fixed subset Mi of the Nwavelength channels and routes them to the de-multiplexer 26. Thede-multiplexer 26 separates this subset into the m separate wavelengthchannels which are then received by their respective receiver 28.Additionally up to p wavelength channels can added to the network bymeans of the transmitters 32 to define connections to other nodes of thenetwork. Since the wavelength of channels added by the node can bere-configured, by wavelength tuning the transmitters, this enablesinterconnection to other nodes to be at least partially re-configured.

The embodiment of FIG. 2 is similar to that of FIG. 1 and likecomponents carry the same reference numerals. However, the opticalde-multiplexer 26 is replaced by a passive multi-way (m-way) opticalsplitter 36 (cascaded fused fiber coupler) which divides radiationapplied to its input equally between its m outputs. Each of the outputsof the splitter 36 contains each of the m wavelength channels dropped bythe broadband drop filter 12 and each is applied to a respectivewavelength selective filter 38. The wavelength selective filter is usedto select a respective one of the m wavelength channels for reception bya respective receiver 28. Conveniently the filters 38 can be wavelengthtuneable enabling a different receiver to be configured to receive agiven wavelength channel. Such a capability is useful in the event offailure of a receiver.

The embodiment of FIG. 3 a is identical to that of FIG. 2 except thatthe passive optical splitter 36 and the multiplexer 30 are arrangedremote from the card 22 on a remote card 40. This arrangement issuitable where it is intended to locate the transmitters and receiversat a position remote from the node. Likewise it will be appreciated thatthe de-multiplexer 26 of FIG. 1 can be remotely located in a likemanner.

In FIG. 3 b, the broadband drop filter 12 is replaced with two or morewavelength selective filters 42, 44 and a passive optical combiner(cascaded fused fiber coupler) 46 which combines the outputs of two ormore filters 42, 44. Such a filter arrangement is suited where thesubset Mi of wavelength channels does not comprise a band of consecutivewavelength channels.

In the embodiment of FIG. 4 a, the node drops the m (λ_(a) to λ_(d))wavelength channels to a sub-ring 50 subtended from the node, which ispart of a main network ring. The subtended ring 50 has a number ofnodes, here two, each of which can drop out one or more of the mwavelengths supported by the sub-ring. Thus, in FIG. 4 a, a firstsub-node 60 of the subtended ring 50 has a drop filter 62 which dropsout a single one (λ_(a)) of the m wavelength channels to a receiver 64.Additionally the node 60 includes a tuneable add filter 66 for adding awavelength channel from a tuneable laser transmitter 68 back onto thesubtended ring. A VOA 70 is placed between the transmitter 68 and theadd filter 66 for reasons discussed previously. The tuneable lasertransmitter 68 can transmit signals at any of the N wavelengthssupported by the N channel network to enable communication with othernodes of the main network.

A second sub-node 80 on the subtended ring 60 includes a furtherbroadband drop filter 82 which can drop out a subset (λ_(b) to λ_(d)) ofthe m channels supported by the subtended ring. The output of the filteris passed to a de-multiplexer 84 which has outputs corresponding to theindividual wavelength channels dropped by the broadband filter 82 (onlyone is shown in FIG. 4 a). Each of these wavelength channels can bereceived by a respective receiver 86.

As in the previous examples, the de-multiplexer 84 can be replaced by apassive optical splitter and tuneable or fixed wavelength filters.However, these are not preferred as they are more lossy than thede-multiplexer arrangement. This additional loss, together with thefiber loss of the subtended ring makes the approach less advantageousthan using a de-multiplexer.

Additionally the sub-node 80 includes one or more tuneable lasertransmitters 88, which have associated VOAs 90, and which provide one ormore channels to a passive optical combiner 92 whose output is coupledback onto the subtended ring by a coupler 94. As before, the transmittercan be tuned to transmit at any of the N channels supported by thenetwork as discussed in previous examples.

FIG. 4 b shows a variant of the subtended ring of FIG. 4 a in which thetuneable add filter 66, used for adding the wavelength tuneable channelonto the subtended ring, is replaced by a passive optical coupler 100.

FIG. 4 c shows a further variant in which the broadband drop filter 12is replaced by two or more separate filters 42, 44 and an opticalcombiner 46 in a manner similar to that of FIG. 3 b.

FIG. 5 shows an example of a thirty two wavelength channel (λ₁ to λ₃₂)WDM ring network comprising four nodes (i=1 to 4) 110, 112, 114, 116.Each node is assigned a fixed subset M_(i) of wavelength channels thatit drops from the ring in this example each drops eight of the thirtytwo wavelength channels. The first node 110 is ascribed the wavelengthchannel subset M₁=λ₁ to λ₈, the second node 112 the subset M₂=λ₉ to λ₁₆,the third node 114 the subset M₃=λ₁₇ to λ₂₄, and the fourth node thesubset M₄=λ₂₅ to λ₃₂. It follows from the foregoing description thateach node includes means (drop filter) for dropping the eight wavelengthchannels ascribed to the node. It also follows that since each node iscapable of adding wavelength channels corresponding to at least any ofthe remaining wavelength channels, each node can communicate with allthe other nodes at any of their wavelengths.

The embodiments of the invention described provide a low cost partiallyre-configurable WDM network in which a fixed number of wavelengthchannels are dropped out at each node and in which each node cantransmit at any of the other wavelengths supported by the network. Sucha network is highly advantageous, particularly in the metro environmentin which the network frequently requires re-configuring. Moreover,re-configuration can be done remotely. For example, the tuneable laserscan be adjusted from a remote management terminal on the networkutilizing, for example, the optical supervisory channel. This greatlyreduces the time delay and cost of re-configuring the network butwithout incurring hardware costs associated with the prior art fullyre-configurable networks in which any of the wavelengths can be droppedat any of the nodes.

Various modifications are possible within the scope of the invention.For example, the subset of wavelength channels dropped at respectivenodes can either be a band of consecutive wavelength channels, as in theexamples described above, or be a grouping of non-consecutive wavelengthchannels. In the case of the latter, for example, every fourthwavelength channel can be selected using an interleaving type filter.

It will be appreciated that the transmitters can comprise a singletuneable transmitter or a plurality of fixed wavelength transmitters. Inthe embodiments described, the transmitters can transmit to all Nwavelengths supported by the network. However, as described this is notessential. Alternatively each transmitter can cover a subset of the Nwavelengths, for example, where the likely network configurations fallinto an appropriate subset of the total.

Typically, the number m of wavelengths dropped at a node will in generalequal the number p of wavelength channels added at the node. However,this may not always be the-case where bidirectional traffic is notalways required, or where the bidirectional traffic is of a differentnature in the two directions.

1-16. (canceled)
 17. A wavelength division multiplex (WDM) opticaltelecommunications network, comprising: a) a plurality of nodesinterconnected by optical fiber waveguides in which communicationtraffic is communicated between the nodes by optical radiation modulatedwith the communication traffic, said radiation being partitioned into aplurality of wavelength channels each channel having a discretenon-overlapping waveband, each node including means for dropping atleast one of the wavelength channels to thereby define a connection tosaid respective node, and each node including means for adding at leastone of the wavelength channels to the network to define anotherconnection to another node of the network; b) at least one of the nodeshaving ascribed to it a fixed subset of the wavelength channels andincluding means for dropping each of the wavelength channels of thefixed subset from the network; and c) wavelength selectable means foradding at least one of at least any of the remaining wavelengthchannels, other than the fixed subset, to the network to define stillanother connection to still another node of the network.
 18. The WDMnetwork according to claim 17, in which every node has ascribed to it arespective fixed subset of the wavelength channels and includes meansfor dropping each of the wavelength channels of the respective fixedsubset from the network; and every node further includes wavelengthselectable means for adding at least one of at least any of theremaining wavelength channels, other than the respective fixed subset,to the network.
 19. The WDM network according to claim 17, in which themeans for adding the at least one wavelength channel includes at leastone wavelength selectable transmitter.
 20. The WDM network according toclaim 19, in which the at least one wavelength selectable transmitter iswavelength tuneable to transmit on any of the wavelength channelssupported by the network.
 21. The WDM network according to claim 19, inwhich the at least one wavelength selectable transmitter comprises atuneable laser.
 22. The WDM network according to claim 19, in which theat least one wavelength selectable transmitter further comprises avariable optical attenuator for controlling power of a respectivewavelength channel.
 23. The WDM network according to claim 17, in whichthe wavelength channels of the fixed subset are contiguous and comprisea band of consecutive wavelength channels.
 24. The WDM network accordingto claim 23, in which the means for dropping the fixed subset of thewavelength channels comprises a broadband drop filter.
 25. The WDMnetwork according to claim 17, in which the fixed subset comprises aplurality of discrete wavelength channels.
 26. The WDM network accordingto claim 25, in which the means for dropping the fixed subset of thewavelength channels comprises at least one fixed wavelength selectivefilter.
 27. The WDM network according to claim 26, in which the at leastone fixed wavelength selective filter drops every Q^(th) wavelengthchannel, where Q is greater than one.
 28. The WDM network according toclaim 17, and further comprising means for separating the respectivewavelength channels of the fixed subset.
 29. The WDM network accordingto claim 28, in which the means for separating comprises ademultiplexer.
 30. The WDM network according to claim 28, in which themeans for separating comprises an m-way splitter and m optical filters.31. The WDM network according to claim 17, in which a number of thewavelength channels that can be added at the respective node is equal toa number m of the wavelength channels dropped at the respective node.32. The WDM network according to claim 17, in which the nodes areinterconnected in a ring configuration.